ok, there bust be an error in the userlogin page

this work:
http://www.michel-nulens.be/zenphoto/index.php?userlog=1&password=mister_1234 (I hand coded it)

this is the url when I tried to login 1 time:
http://www.michel-nulens.be/zenphoto/index.php?userlog=1&p=password#

do you see the difference?

I have no idea. Sadly my collegue sbillard who is the expert on login stuff is away for a week.

In the meantime please review your server error log and also the debug log within /zp-data of your install.

So are you tryig to provide the password as a query parameter to the form? That will simply not work. Password forms require the data to be Posted.